Road salt is seen to be the enemy, but the precise role of chloride
ion in the corrosion process is not fully understood

Corrosion results from the chemical or electrochemical reaction
between a material and its environment. All materials (metals, plastic,
composites, ceramics) corrode to some degree. This article will consider
only metallic corrosion and oxidation, although many of the
surface-analytical techniques discussed can be applied to study the
degradation of non-metals.

Metallic corrosion can take many forms, e.g. uniform attack,
localized corrosion or stress corrosion cracking. The structure and
composition of the |passive' oxide film which forms in aqueous in
environments on all metals except gold is important in governing
corrosion resistance. Metals become passive as a result of this oxide
which can form when the potential is raised above a critical value, as
indicated for metallic iron, Fig. 1. Once this protective film breaks
down, corrosion is initiated and a corrosion product forms.

Modern surface-analytical techniques can provide useful information
regarding the nature and chemical composition of these passive oxide
films, leading to a better understanding of the corrosion processes that
occur on an atomic scale. These techniques also provide useful
information regarding transport processes in oxides grown at high
temperature. Surface techniques can be of two-types: ex situ methods
(where samples are removed from solution or a reaction vessel and placed
in ultra-high vacuum systems), such as Auger electron spectroscopy (AES), secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectroscopy (XPS), and in situ methods, such as infrared (IR) and Raman
spectroscopy, Table 1 shows a summary of the first three techniques
together with the information they provide. These three techniques have
been widely used to examine passive films and corrosion products,
whereas in situ techniques have been used more to study the role of
corrosion inhibitors. This article will also concentrate on ex situ
methods, and will illustrate the application of the various techniques
to corrosion and oxidation research studies performed in our laboratory.
Over the years our efforts have focused on the nature of passivity, its
breakdown by aggressive ions such as chloride, and the transport
processes which take place during oxide growth at high temperatures.

The |passive' oxide

film on iron

The nature of this common oxide film has been the subject of
investigation since the days of Michael Faraday. While SIMS, Auger, XPS,
etc., are ideally suited to examine the 1.6-nm-thick film, its precise
nature and chemical composition are still a matter of some controversy.
Our view is that the film is crystalline and consists, of an inner
magnetite ([Fe.sub.3][O.sub.4]) layer and an outer magnetite
([gamma]-[Fe.sub.2][O.sub.3]) layer, Fig. 1, of which the outermost portion is cation deficient.

Others consider the film to be an amorphous gel with bridging
di-oxy and di-hydroxy bonds. Surface-analytical data from our laboratory
support the [Fe.sub.3][O.sub.4]/[gamma]-[Fe.sub.2][O.sub.3] duplex
model[1]. In particular, SIMS can be used to detect the presence of
H-containing species and the lack of hydroxyl ions within passive films
has been confirmed in such experiments[2]. Figure 2 shows experimental
SIMS data for a passive film together with |dry'
[Fe.sub.2][O.sub.3] and |wet' FeOOH (lepidocrocite) standards. The
film is removed by ion sputtering and the [FeO.sup.+] and [FeOH.sup.+]
signals are measured through the film (depth profiling). The profile for
the passive oxide film is very similar to that for the |dry'
[Fe.sub.3] [O.sub.2] standard until the oxide/metal interface is reached
after about

7 minutes of sputtering, (For |wet' FeOOH. the [FeOH.sup.+]
signal is always higher than that of [FeO.sup.+.]) The hydroxyl content
within the film as calculated from the SIMS data is zero ([+ or -]
0.1%); only a fraction of a monolayer of OH is adsorbed on the oxide
surface as seen by the initially higher [FeOH.sup.+] signal. Conversion
electron (back-scattered) Mossbauer spectroscopy (EMS) is much more
surface sensitive than the conventional transmission technique and has
been used to examine passive oxide films on iron. The data can be
interpreted in terms of the film being a small particle size crystalline
oxide, and both CEMS and complementary XPS data support the model that
the passive film resembles [gamma]-[Fe.sub.2] [O.sub.3] [3].

Questions are sometimes raised regarding possible changes which may
occur in passive films when specimens are removed from solution and then
pumped in an ultra-high vacuum system for SIMS. Auger or XPS analysis.
Performing electrochemical experiments in [O.sup.18]-containing solution
and then monitoring the [O.sup.18] level of films using SIMS is a good
method to determine the air stability of passive films. If films are
unstable, they will pick up [O.sup.18] from the air, and their
[O.sup.18] content will be less than that of the solution enrichment.

Passive films on iron (except at very low potentials in the passive
range shown in Fig. 1) are found to be stable in the air. Rather
surprisingly, films on iron-chromium alloys (more like stainless steels)
do change on removal from solution, and the extent of change increases
with increasing chromium content of the alloy [4]. [O.sup.18] /SIMS can
also be used to indicate whether, for example, airformed films can be
removed by a cathodic treatment to leave a bare metal surface prior to
performing an electrochemical experiment to produce the passive films
described above.

The technique can also be used to study the mechanism of growth of
films at different potentials. Films can be formed at one potential in
the passivee range, Fig. 1 and then the potential increased to a higher
value still in the passive region, for additional growth in an
[O.sup.18]-enriched solution. A subsequent SIMS profile through the
oxide shows the [O.sup.18.] to be a maximum at the outer surface and
then decreases throughout the entire thickness of the film [1]. This
result indicates that the additional oxide has grown by inward oxygen
transport through the film, likely via both lattice and grain boundary
sites. (Oxide growth studies using [O.sup.18]/SIMS will be described
later in more detail for high temperature corrosion.)

Role of chloride ion in

pit initiation

From a practical viewpoint, the influence of road salt on
automobile corrosion has dramatic consequences, yet the precise role of
the chloride ion in the corrosion process is not fully understood. For
example, there is considerable disagreements as to whether chloride ion
is incorporated into passive films as a precursor for the initiation of
pitting corrosion of the metal. We have used both AES and SIMS to
determine the extent of any chloride ion incorporated into films formed
on nickel, iron and iron-chromium alloys.

As seen by the Auger profile, Fig. 3(a), passive films formed on
nickel in 1M chloride solution are found to contain up to 4% chloride
ion [5], whereas films formed on iron do not incorporate chloride ion
even when formed in chloride-containing solution [6]. Chloride ion is
also incorporated into films on iron-chromium alloys after electron
polishing a surface in a perchloric/acetic acid mixture.

Figure 3(b) shows the Auger profile of such a film. Quantitative
layer-by-layer Auger analysis [7] shows that chloride is distributed
over the outer six layers of the 10 layer-thick oxide. This
quantitative analysis is compared to the experimental data, in Fig.
3(b). From the analysis it is concluded that the inner part of the film
is chromium-rich and the outer part is iron-rich. The presence of
incorporated chloride in oxide films influences the open-circuit
breakdown behavior of both nickel and iron-chromium alloys.

However, experiments with nickel show that the incorporated
chloride is not a precursor for pit initiation, and that nickel is more
resistant to pitting when chloride is incorporated in the film. The
precise events which give rise to breakdown of passive films in chloride
solution are still being investigated by many groups around the world.

The performance of materials at high temperature is often
controlled by the protective qualities of the surface oxide which forms.
It is important, therefore, to understand the oxide growth processes in
order to develop more corrosion-resistant coatings or alloys. SIMS is
ideally suited to examine these growth processes, in particular the
extent of inward oxygen transport compared with outward cation transport
through the oxide scale [8]. By this technique, in addition to examining
to [O.sup.18] and [O.sup.16] signals, the negative polyatomic species M
[O.sup.18.sub.2], M [O.sup.16.sub.2] and the isotopically mixed M
[O.sup.18] [O.sup.16] can also be analyzed after sequential oxidation of
a material in [O.sup.18.sub.2] and [O.sup.16.sub.2]. An example of such
a polyatomic SIMS profile is shown for a thick oxide grown on a (100)
single crystal chromium surface at 825[degrees] C, Fig. 4(a), (1.1 [mum]
of oxide was produced in [O.sup.18.sub.2] followed by an outer 0.3 [mum]
in [O.sup.16.sub.2]). It is clear from the profile that the latter oxide
is on the outside of the scale, showing that the major transport process
during oxidation is outward chromium diffusion.

However, the Cr [O.sup.18] [O.sup.16] profile is asymmetric about
the [O.sup.18]-oxide/[O.sup 16]-oxide interface with an obvious oxygen
penetration into the inner oxide. This is considered to be due to some
inward oxygen diffusion down oxide grain boundaries. The amount of oxide
created in this way within the existing [.sup 18O]-containing film, is
however, small, accounting for only 1% of the total new oxide. There is
also a small rise in the [Cr.sup 18 O.sup 16 O] and [Cr.sup 16 O.sub 2]
signals in the vicinity of the metal/oxide interface which corresponds
to the zone where the original 2-nm-thick, [O.sup 16]-oxide film formed
during surface pretreatment would be situated.

Expansion of the [Cr.sup 16O.sub 2] profile confirms a small but
clearly distinct peak in this region, Fig. 4(b), for polycrystalline chromium oxidized at 825 [degrees] C. This result demonstrates that the
[sup.18 O]/SIMS technique is sufficiently sensitive to allow a
relatively thin layer of oxide to be detected underneath a very much
thicker external layer. This thin prior oxide film, in fact, serves as
an inert marker confirming oxide growth primarily by outward cation
diffusion from the metal to the outer oxide surface, and formation of
new oxide mainly on top of the existing scale.

The addition of small amounts of so-called reactive elements such
as cerium and yttrium increases the high-tempperature oxidation
resistance of chromium and iron-chromium alloys. We have applied very
thin (4nm) reactive element coatings by sputtering ceria (CeOsub.2) or
ytttria ([Ysub.2 O sub.3]) directly onto chromium or iron-chromium
substrates, and have used SIMS to determine the location of the reactive
element within the oxide scale and to study its effect on transport
processes [9]. For examples, SIMS profiles of a thin oxide (76 nm-thick)
formed after short-term oxidation at 900 [degrees] of iron-26% chromium
coated with 4 nm of ceria are shown in Fig. 5(a).

The cerium in the oxide layer is detectable and its maximum signal
is observed to be away from the alloy/oxide interface. With prolonged
oxidation the location of the cerium maximum moves out further towards
the oxide/gas interface.

If the ceria coating can be considered to be a stationery marker,
its location towards the oxide/gas interface indicates that inward
oxygen diffusion is becoming more prominent in governing oxide growth.
[Osup.18]/SIMS experiments confirm that oxygen transport is occuring.
Figure 5(b) shows data for a 0.36 [micro]m-thick oxide formed
sequentially in [16 sup.O].2 and then in [sup.18 O sub.2] at 900
[degrees] C. In this longer-term experiment the location of the cerium
maximum is now in middle of the scale. There are three [O sup.18] maxima
([Cr sup.53 O sup.18 sup.+]): at the gas/oxide interface, within the
scale coincident with the cerium maxima, at the alloy/oxide interface
showing that oxygen diffusion has occurred during oxidation.

Examination of SIMS data. Fig. 5(b), in the form of the fraction of
[O sup.18] associated with the cerium- and chromium-bearing species
through the scale shows that a higher fraction is associated with the
cerium-bearing species. If the cerium in the scale is located at oxide
grain boundaries, the association of [O sup.18] with cerium strongly
suggests that transport of [O sup.18] through the scale occurs along
grain boundariees. Therefore, coating iron-chromium alloys (or chromium
metal) with a small amount of ceria causes a change in oxide growth
mechanism from predominantly cation diffusion for uncoated material,
Fig. 4(a) to predominantly anion diffusion via oxide grain boundaries,
Fig. 5(b).

SIMS lacks the spatial resolution to determine the precise location
of the cerium or its form and distribution within the chromium oxidee
layer. However, techniques such as transmission electron microscopy
(TEM) with electron energy loss (EELS) analysis [10] can be used to
identify any cerium-chromium oxide phases formed at oxide grain boundary
regions. Figure 6 shows both an EELS image (a) and bright field image
(b) from the same area of a 44 nm-thick oxide formed on ceria-coated
iron-26% chromium at 900[degrees] and stripped from the substrate.
Figure 6(a) shows the distribution of cerium-containing oxide as the
light colored regions.

X-ray analysis in TEM shows the oxide particles to have a cerium to
chromium ratio

1 suggesting that the particles are the oxide [CeCrOsub.3], and
this has been confirmed by electron diffraction measurements [11]. It is
likely that these particles and, perhaps cerium ions, retard the grain
boundary diffusion of chromium cations to the extent that the mobility
of oxygen anions along these grain boundaries exceeds that of the
cations.

EELS microscopy provides complementary information to AES and SIMS
data regarding metal corrosion and oxidation processes because of its
high spatial resolution. One of the main disadvantages of techniques
like AES and SIMS is their relatively poor spatial resolution compard
with TEM. However, developments continue to improve the resolution of
surface techniques. For example, with a liquid metal ion gun, it is now
possible to obtain SIMS images with a spatial resolution better than 50
nm. This can be imagined from the SIMS images, Fig. 7, taken from an
oxide formed at 1100 [deegrees] C, first in [sup.16 O sub.2] and then in
[sup.18 O sub.2], on an iron-25% aluminum alloy.

black areas those where patches of oxide have spalled off. As we
profile through the oxide towards the alloy surface, the white [O
sup.18] rich patches disappear, some of the lines disappear and the low
level in the grey areas in maintained. In addition, some areas of oxide
containing 50% [O sup.18] appear as the oxide/alloy interface is
approached. (Figure 7(b) shows an [O sub.16] image taken near the
oxide/alloy interface indicating the high level of [O sup.16]. The [O
sup.18] distribution is localized and non-uniform, and the [O sup.18]
regions near the oxide/alloy interface are consistent with oxygen
short-circuit diffusion and formation of new oxide grains at the alloy
surface. Clearly, the SIMS images illustrate that growth mechanism of
alumina ([alpha]-[Alsub.2 O sub.3] is much more complex than simply
oxygen grain boundary diffusion.